1. Field of the Invention
The present invention relates to electrically conductive paste and an electrically conductive circuit member of a printed circuit board for use in an optical device such as cameras, office machines, audio products, OA equipment, home electric products, meters and communication devices.
2. Description of the Prior Art
Hitherto, electrically conductive circuits have been formed in a solid phase-gas phase system on an insulating substrate by an evaporation method, an ion plating method, a CVD method and a sputtering method. Electrically conductive circuits have also been formed in a solid phase-liquid phase system by a plating method, a metal soldering method and a metal-oxide solder method. Furthermore, these circuits have been formed in a solid phase-solid phase system by a DC voltage application method and a pressing and heating method.
However, the above-described conventional methods of forming a circuit encounter the following problems:
The evaporation method and the sputtering method employed in a solid phase-gas phase require use of an expensive vacuum apparatus. Further, the type of the substrates which may be employed are limited, causing problems such that the overall cost cannot be reduced and mass production cannot easily be performed. Furthermore, since the circuit is formed at high temperature, use of the above-described methods is limited to substrates possessing sufficient heat resistance.
It is necessary for each of the plating methods, the metal soldering method and the metal-oxide mixture soldering method to make the thickness of the plating layer, which serves as the pattern, 18 .mu.m or greater. Therefore, it takes an extended time to complete the film formation, preventing satisfactory production levels. Therefore, the overall cost cannot be reduced. Furthermore, the solder method is arranged in such a manner that silver powder, amorphous carbon powder or graphite powder is mixed with a binder such as phenol resin, epoxy resin, polyester resin or acrylic resin so that a desired pattern is screen-printed via a mask. However, that printing method encounters a problem in that the obtainable resolution is 150 .mu.m or lower. Therefore, it cannot be employed in an apparatus such as a lap top computer because the size, weight and thickness cannot be reduced, or the circuit realized in the form of a card. Further, another problem arises in that the displayed quality is unsatisfactory and reproducibility is insufficient. For example, bleeding, blurs and insufficiency of the functional component, which take place in the printed film cannot be prevented. Finally, since the screen mask pattern can be undesirably plugged, the quality will be deteriorated.
In addition, the thickness of the formed film cannot be equalized if the squeezing pressure or the squeezing speed is changed. In consequence, sharp forms cannot be realized and the reproducibility also becomes unsatisfactory, causing reduced quality of the product.
Furthermore, a satisfactory positional accuracy cannot be obtained in the printing pattern, and the high temperature of 150.degree. C. or higher is necessary to harden the film. Therefore, the substrate will become warped or twisted, and a dimensional problem exists, causing deterioration of the ability and efficiency of connection to the adjoining parts when assembled. As a result, a critical problem arises in that the cost cannot be reduced and a satisfactory efficiency in the mass production cannot be realized.
Furthermore, since the above-described paste contains a toxic organic solvent, a risk of contaminating the working space or fire hazard cannot be overcome.
The high melting metallizing method is usually employed in the solid phase-solid phase system. However, that method forms the circuit at high temperature similarly to the solid phase-gas phase method. Therefore, the application is limited to a substrate such as a ceramic plate which reveals sufficient heat resistance and dimensional stability.